1. Field of the Invention
The invention relates to vehicle emission control devices which store a constituent gas of the engine-generated exhaust gas during a first engine operating condition and which release previously-stored constituent gas during a second engine operating condition.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-storing efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity. Unfortunately, however, the use of such a fixed device capacity necessarily requires a larger device, because this prior art approach relies upon a partial, e.g., fifty-percent NOx fill in order to ensure retention of all engine-generated NOx.
It is an object of the invention to provide a method and system by which to optimize the operation of a vehicle emission control device through improved open-loop control of device fill and purge events.
Under the invention, a method and system is provided for controlling a fill time cycle and a purge time cycle of an emission control device that receives exhaust gas generated by an internal combustion engine, wherein the device is filled with a constituent gas of the exhaust gas during a first engine operating condition and is purged of constituent gas during a second engine operating condition. The method includes selecting, in a normal open-loop mode of operation, a fill time and a purge time from a set of predetermined values as a function of an engine operating condition; and cyclically filling and purging the device based on the selected fill and purge times. The method further includes determining, after a predetermined number of fill and purge cycles, a first value representative of current total device capacity to store the constituent gas, and a second value representative of a quantity of oxygen stored in the device; and updating at least one of the predetermined values as a function of the first and second values. Preferably, the selected purge time is optimized only upon operation of the engine at an operating point corresponding to a limited number of engine operating conditions.
In accordance with a feature of the invention, an exemplary method includes periodically optimizing a selected purge time associated with a selected fill time by filling the device for the selected fill time; purging the device for the selected purge time; generating a third value representative of oxygen concentration present in the exhaust flowing through the device during a predetermined sampling period which includes at least an end portion of the purge time, for example, using an oxygen sensor; comparing the third value to a predetermined reference value, wherein the reference value is based on an optimized value for device capacity utilization; and generating an adaption value for modifying the selected purge time as a function of any error between the third value and the reference value.
By way of example only, in an exemplary method for practicing the invention, the step of determining the first value representative of current total device capacity includes filling the device to a saturation level; generating a fourth value representative of an oxygen concentration present in the exhaust flowing through the device; generating a first error value as a function of the fourth value and a predetermined reference value; and determining an actual purge time necessary to purge the device when the device is filled to the saturation level using the first error value. Similarly, in an exemplary method, the step of determining the second value includes, in sequence, partially filling and purging the device to a first sub-optimal level and a second sub-optimal level over different time periods; and for each sub-optimal filling and purging, generating respective values representative of the oxygen concentration in the exhaust passing through the device. The step of determining the second value in the exemplary method further includes generating respective error values based on the sub-optimal fill oxygen-concentration values, and a predetermined reference value; and determining a respective actual purge time necessary to purge the device for each sub-optimal fill based on the respective error values, whereupon the desired second value is determined as a function of the sub-optimal fill times and the actual purge times.
In accordance with another feature of the invention, in an exemplary method, the adaption value is generated as a function of either the generated third value if the third value is not greater than the reference value, and the adaption value is generated as a function of a length of time that the third value exceeds the reference value if the third value exceeds the reference value. In this exemplary method, the step of generating the adaption value further includes linearly extrapolating the third value in proportion to the measured oxygen level when the generated value is below the reference value.
From the foregoing, it will be appreciated that, under the invention, a method and system are provided for controlling the filling and purging of an emission control device used to reduce vehicle tailpipe emissions of a constituent gas, includes operating in a normal open-loop mode by determining fill times and purge times based on engine operating conditions and corresponding values stored in a memory, while periodically optimizing a purge time associated with a selected fill time. After a predetermined number of fill and purge cycles have been performed, a first value is determined which is representative of current total device capacity, and a second value is determined which is representative of a quantity of oxygen stored in the device. At least one purge time value is updated as a function of the first and second values.
Thus, the invention provides an open-loop system and method for controlling the filling and purging of an emission control device in which a more accurate open-loop determination of the device""s instantaneous capacity to store constituent gas of the engine-generated exhaust gas is obtained through periodic adaption of open-loop values in response to a determination of a purge time associated with oxygen stored within the device during lean engine operation, and of a device saturation purge time. The resulting adapted open-loop values for device fill and purge times provide improved control of tailpipe emissions while further operating to minimize fuel consumption associated with purging of the device.